I've read that James Dean was a closet case and frequented underground sex clubs. Also that he liked to be used and abused, and was into pain and let people burn him with hot cig butts.
I think the first time that legend was actually published was in Kenneth Anger's
Hollywood Babylon; there's also that pic of him in the tree jerking off in that book.
His entire short life has been mythologized to the point where the truth will never really be known. My guess is that he was probably a deeply closeted bisexual with extremely self-destructive impulses, resulting in his premature death. He worshiped Brando (another bisexual), who by all accounts wanted nothing to do with him. He was known for tardiness, moodiness and an over-all unprofessional attitude; his hygiene has often been remarked upon as having been deplorable.
Given the tight leash Hollywood studios kept on their stars, he probably would not have had much of a brilliant career, despite the promises of his earliest films. Had he lived, he'd have probably found fewer offers as time passed; perhaps he'd have gone into TV or back to NYC and tried his hand on stage (though he wasn't known for his vocal projection, and mics on Broadway are a newish thing). One way or another, he'd have probably self-destructed before the mid-sixties.
As regards his equipment: he was a small guy, about my build and body-type. Though there's no way of being certain, he always struck me as smallish-average who didn't bring a lot of enthusiasm or stamina to bed: in other words, probably a disappointing lay. I'll also guess that he was uncut and had smaller balls in a tight sac.